In the San Joaquin Valley (Calif.), for instance, the estimated daily production of agricultural drainage water (ADW) is over 1.3 million m3. ADW was considered waste that required disposal into solar ponds, the San Joaquin River, and the Pacific Ocean. The disposal rather than the reuse of ADW is still, however, the on-going practice in most areas of the San Joaquin Valley. Such a practice has led to costly environmental and ecological problems.
Until recently, the Integrated On-Farm Drainage Management (IFDM) system is experimented in some areas of the San Joaquin Valley. FIG. 1 shows the stages of the IFDM system. The IFDM system allows partial and sequential recycle of ADW to irrigate crops of progressively increasing salt tolerance. Farm operation is divided into three salinity areas: (1) the non-saline zone in which good quality irrigation water is used (e.g., for vegetables); (2) low-salinity zone in which recycled ADW from the non-saline zone is used (e.g., for cotton and grasses); and (3) moderate-salinity zone in which recycled ADW from the low-salinity zone is used (e.g., for various salt-tolerant trees, grasses, and halophytes). Once the progressively recycled ADW approaches intolerable salinity levels for the targeted crops, it will then be transferred via sumps into solar ponds or solar basins or water treatment facilities. In the short-term, the IFDM system partially manages ADW. However, the long-term effects on soil-groundwater (increase in salinity and toxicity), and humans-livestock (toxicity) could rise again as significant problems. In addition, the accumulation of untreated ADW in solar ponds and basins is still a problem.
Table 1 reveals salinity variations in ADW from farm operations that are no longer suitable for crops irrigation (Red Rock Ranch, Calif.). The salinity levels of the stored ADW range between about 11,000 mg/L and 300,000 mg/L. The bulk of salts are in the forms of sodium sulfate and sodium chloride. The bulk of scale species is in the form of calcium sulfate. Toxicity is largely manifested in the forms of selenium, and to a lesser extent in the forms of boron and transition metals (copper, iron, manganese, arsenic, and aluminum). The U.S. Environmental Protection Agency (EPA) ambient freshwater aquatic life continuous concentration criterion for selenium is 5 μg/L. Selenium concentrations in ADW are appreciably higher than 5 μg/L. Selenium at elevated concentrations behaves as a potent toxicant to waterfowl, plants, livestock, and humans.
The abundance of both solar radiation and ADW to absorb solar radiation may justify the generation of a large number of solar ponds and basins to store ADW. However, solar ponds and basins: (1) allow the gradual concentration rather than the immediate treatment of ADW to remove toxic species (e.g., selenium, metals, and boron) and recover salts (e.g., sodium sulfate and sodium chloride); and (2) waste the naturally evaporated ultra-pure water. Such solar evaporators serve as long-term salt-sinks for AWD that represent potential hazards to wildlife and groundwater quality.
Pressure-driven membrane systems such as Reverse Osmosis (RO) and Nanofiltration (NF) as well as conventional thermal-driven systems such as Multi-Effect Vapor Compression (MEVC) and others can be used to produce potable water from ADW. RO and NF systems are salinity-sensitive, and therefore, their power consumption is directly related to salinity. Since the selectivity of RO membranes is nearly ions-blind, the application of such membranes is generally limited to saline streams with osmotic pressures of about 500 psi. On the other hand, NF membranes are ions-selective that highly reject polyvalent anions and allow monovalent anions to pass through the membrane. NF membranes can thus be applied to saline streams with relatively high osmotic pressures, if the predominant salt in such streams is a monovalent (e.g., sodium chloride) rather than a polyvalent (e.g., sodium sulfate) salt. However, the predominant salt in ADW from the San Joaquin Valley is sodium sulfate. As such, the application of both RO and NF is almost equally limited to ADW streams with osmotic pressure of about 500 psi.
Thermal-driven desalination systems are generally salinity-insensitive, and thus, they are applicable to almost any saline stream. However, conventional thermal-driven systems are energy intensive, and their power consumption is not related to salinity. For instance, there are almost insignificant differences in power consumption between treating an ADW stream with 10,000 mg/L or a stream with 100,000 mg/L. Although the ruggedness of conventional thermal-driven desalination minimizes stages of pretreatment compared to the delicate RO and NF membranes, their capital cost is prohibitive. As such, they are economically unattractive to treat ADW.
If the average energy requirement to operate pressure-driven or thermal-driven desalination systems is assumed to be 10 kWhr/m3 (about 50% lower in the case of well optimized RO, and about 50% higher in the case of well optimized MEVC), then the capacity of the required power plants to supply this energy to treat the daily generated ADW would be about 2,000 megawatts. In addition, ions paring of calcium-sulfate, strontium-sulfate as well as the existence of other scale-prone species in ADW could significantly impair the performance of such systems (higher operating cost). Further, neither pressure-driven (RO or NF) nor thermal-driven (MEVC) as a stand alone provides a zero or a near-zero discharge system, and thus the disposal of the concentrate stream is a critical issue.
Salt-gradient solar ponds (SGSP) are a cheap method for storing and using concentrated saline water to produce power (thermal and/or electrical). The functionality of SGSP is different from typical solar ponds. As shown in FIG. 2, the SGSP consists of three distinct layers: (1) surface zone (upper layer); (2) gradient zone (middle layer); and (3) heat-sink zone (lower layer). The lower zone contains a hyper-saline water (salinity exceeds 360,000 mg/L) that absorbs sunlight and serves as a heat sink. The temperature range of the lower zone is typically maintained between 340 and 370K to stabilize the gradient zone and prevent boiling. The middle zone contains water with variable salinity (approaches 100,000 mg/L) and serves as an insulating layer for the lower zone. The upper zone contains low salinity water (about 10,000 mg/L) or potable water that serves as a controlling parameter to efficiently operate the SGSP. The temperature of the upper zone is near or slightly below ambient temperature. Power can be produced by circulating hot hyper-saline water from the lower zone, for instance, to a Rankine cycle turbine to generate electricity or to a heat exchanger to provide thermal energy.
Apparently, the solution for ADW must be centered on two main issues. The first issue is to provide affordable energy resources to operate ADW treatment technologies. A manageable number of SGSP could serve the long-term energy requirements using the abundance of solar radiation combined with discarded ADW, or with makeup aqueous streams that contain the selectively recovered salts (e.g., sodium sulfate) from ADW.
The second issue is to immediately treat the generated ADW, rather than the on-going practice of “accumulate, contaminate, and then remediate” the aged and concentrated ADW. This would reduce the massive number of solar ponds and basins, minimize environmental impacts, and speed-up the recovery and production of salts. However, immediate treatment of ADW requires the use of economically-oriented, zero-discharge technologies that meet three critical objectives: (1) selective removal of sulfate scale and/or toxic species; (2) production of reusable water that at least meets agricultural water quality; and (3) segregation and extraction of worthy economic values from the bulk of separated salts (sodium sulfate and sodium chloride). This patent provides novel processing methods to achieve such objectives.
The innovative methods in this patent are divided into three main processing groups. The first group is based on the compressed-phase precipitation (CPP) process to treat ADW at all salinity levels. The second group is based on pressure-driven membranes (RO and NF) in conjunction with CPP to treat low salinity ADW. The third group is based on thermal-driven membranes (membrane distillation, MD) in conjunction with CPP to treat ADW at all salinity levels.